Sintering and grain growth of ultrapure alumina

The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900°C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina ( 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.     

Effect of surface roughness on the subsequent growth of MgO nanowires

We demonstrated the variation of product morphology by varying the underlying surface characteristics, which were controlled by predeposition annealing. The surface roughness of the underlying In2O3 layer affected the resultant product morphology, indicating that rougher substrate favored the formation of one-dimensional (1D) structures. The obtained MgO nanowires were a single-crystal cubic structure. The PL measurement with a Gaussian fitting exhibited visible light emission bands centered at 2.26 eV and 2.86 eV.     

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.     

Effect of sintering on TiO2-impregnated alumina foams

The effect of sintering on the bulk properties, morphology and phase composition of ultralight Al₂O₃ foams impregnated with TiO₂ was investigated in comparison with pure alumina foam in the temperature range of 900–1600°C in air. Impregnation was carried out by immersion of pre-sintered alumina foam in a sol of titanium isopropoxide-acetylacetone complex. The changes of the foam linear shrinkage, effective density and porosity were studied along with morphological evolution and relationship between these properties was demonstrated. Titania impregnation increased the linear shrinkage (LS) during sintering by a maximum of 5% relative to pure alumina foams. The change of LS and weight loss of TiO₂/Al₂O₃ foams lead to a final density of 0.19 g/cm³ and porosity of 95%. The initial coating was found to develop a mosaic structure due to early shrinkage of the coating. After sintering at 1600°C the coating reacted with the underlying Al₂O₃ surface and became uniformly distributed. Finally, it was shown that the reacting TiO₂ layer formed the tialite (Al₂TiO₅) phase below 1400°C. This Tialite coating remained intact under 1200°C without stabilizers.     

Effect of heating rate on the exaggerated grain growth during the sintering of Sr-hexaferrite

The sintering of Sr-hexaferrite has been carried out with slow and fast heating rates and the resultant variation of exaggerated grain growth investigated. The exaggerated grain growth was observed to occur in the presence of a liquid phase, and the fast heating resulted in a high initiation rate for exaggerated grain growth.     

Effect of surface roughness on dip-pen nanolithography

In the present work, five gold thin films with various surface roughnesses were prepared by sputtering and the influence of the surface roughness of gold substrate on dip-pen nanolithography (DPN) was studied using 1-octadecanethiol (ODT) and poly(vinylidene fluoride-trifluorethylene) [P(VDFTrFE)] as inks. It was shown that surface roughness influences both the contrast in lateral force microscopy (LFM) images and the transport rate of ink. Surfaces with less roughness give good contrast in LFM images, while rough surfaces give poor contrast. The transport rate of ink increases as the roughness decreases; however, the extent of the influence is strongly ink-dependent.     

Effect of sintering temperature on the grain growth and electrical properties of barium zirconate titanate ferroelectric ceramics

In the present work, structural, dielectric and ferroelectric properties of barium zirconate titanate ferroelectric ceramics have been investigated. The specimens were synthesized using a solid state reaction technique. The XRD analysis reveals that the synthesized compound was formed with no secondary phases. As the sintering temperature increases from 1,200 to 1,300 °C, the average grain size is observed to increase from ~0.39 to ~6.15 μm. The dielectric measurements as a function of temperature show a decrease in Curie temperature (T_C) on increasing the sintering temperature. The decrease in Curie temperature is attributed to the substitution of Zr⁺⁴ whose ionic radius is larger than Ti⁺⁴. A large increase in the dielectric constant with the increase in grain size is observed. The remanent polarization is also observed to increase with the increase in grain size.     

Multiple grain growth events in liquid phase sintering

Sintered tungsten heavy alloys consist of a solidified liquid alloy matrix phase which interpenetrates a solid tungsten skeletal structure. A consequence of liquid phase sintering is considerable grain growth while the compact densifies. The driving force for grain growth is a decrease in the interfacial surface energy, and the process itself is the combined result of liquid diffusion, solid diffusion, and vapor diffusion if porosity is present. In this study, we utilized microgravity sintered samples to avoid solid-liquid segregation to study the multiple diffusion processes. Coupled with the diffusion event through the liquid phase, there is simultaneous solid-state sintering such as coalescence. The dihedral angle determines the contiguity and the grain growth rate. The liquid diffusion grain growth rate constant is at least one order of magnitude larger than the solid diffusion grain growth rate constant. As composition changes, the ratio of grain growth contributions from these three components also changes, which, in turn, causes grain size, grain size distribution, and contiguity variations.     

Effect of surface mechanical attrition treatment of titanium using alumina balls: surface roughness,contact angle and apatite forming ability

The effect of surface mechanical attrition treatment (SMAT) of commercially pure titanium (CP-Ti) using 8 mm ? alumina balls was studied. SMAT induced plastic deformation, increased the surface roughness, reduced the grain size and decreased the contact angle (from 64° to 43°) with a corresponding increase in surface energy (from 32 to 53 mJ/m²). Untreated CP-Ti and those treated using alumina balls for 900 s reveals no apatite growth until the 28th day of immersion whereas those treated for 1800 and 2700 s exhibit apatite growth in selective areas and the extent of growth is increased with increase in immersion time in SBF. The study reveals that SMAT using alumina balls is beneficial in imparting the desired surface characteristics, provided the surface contamination is limited, which would otherwise decrease the apatite forming ability.     

Effect of sintering schedule on grain size of oxide ceramics

The sintering and grain size of sub-micrometric CeO₂ and ZrO₂ + 3 mol%Y₂O₃ prepared by injection moulding and cold isostatic pressing were studied. Using combinations of the ceramic powders used and shaping methods, three types of bodies were prepared that were characteristic by their own green body microstructure. Their sintering behaviour was described with the aid of high-temperature dilatometry. Each type of specimen was then sintered to two levels of final density (higher than 98% t.d.), for each density always via two or three different firing modes differing in temperature and holding time. In the ZrO₂ specimen it was verified statistically that the final grain size was the same irrespective of whether the given final density was obtained by sintering for a longer period at a lower temperature or for a shorter period at a higher temperature. This finding did not hold for the injection moulded CeO₂ cylinders, which contained cavities in the order of millimetre. When these defects in the structure of CeO₂ ceramic were removed, it was verified statistically also for this material that the final grain size was a function of the density obtained and not the temperature cycle with which this density was obtained.     

Computer simulation of grain growth by grain boundary migration during liquid phase sintering

From many experiments with mixtures of small and large particles, it can be concluded that during liquid phase sintering, smaller particles partially dissolve and a solid phase precipitates on the larger particles. Therefore, the number of smaller particles decreases due to coarsening. The growth rate can be controlled either by the solid-liquid phase boundary reaction or by diffusion through the liquid phase. This dissolution-reprecipitation process leads to further densification by rearrangement of smaller and larger particles. The microstructure may change either by larger particles growing during the Ostwald ripening process or by shape accommodation. In this study, two-dimensional simulation of grain growth by grain boundary migration based on such a physical and corresponding numerical modeling of liquid phase sintering was considered. The simulation method developed is based on the defined submodels for model system definition, for solution-precipitation, and for grain coarsening process.     

Three-dimensional computer modeling of grain growth and pore shrinkage during sintering

This paper deals with modeling of grain growth and pore shrinkage during sintering. The simulation takes as the theoretical basis the curvature of the grain boundaries and the pore surfaces and their mobilities. The movement of the grain boundaries is calculated for each grain separately using the flux equation for grain growth j=c_k(1/r₂−1/r₁), c_k being a rate constant, r₁ and r₂ effective radii for each grain boundary. The modification of these boundaries in a micro structure is calculated in two and three-dimensional space starting with a Voronoy mosaic at the beginning of sintering. Two kinds of pores are distinguished: sphere like pores situated between two grains and non spherical pores between at least three grains. The results of several computer runs are shown and discussed.     

Influence of the ion incidence angle of alumina surface cleanness,roughness and topography

Polycrystalline ceramic specimens of 96% sintered alumina and mechanically polished 99.5% alumina were bombarded by Ar^ü ions at an applied voltage of 7 kV at various incident angles from 0 to 1.4 rad (0 to 80°) and at a beam current of about 70 A. The influence of the incidence angle on alumina surface cleanness, roughness and topography was investigated.     

硅溶胶添加对氧化铝多孔陶瓷烧结性能的影响

以煅烧α-Al2 O3粉末为原料,硅溶胶为高温结合剂,羧甲基纤维素钠为成型黏结剂,通过混料、困料、模压成型、高温烧结等工序制备氧化铝多孔陶瓷,利用SEM和XRD对多孔陶瓷微观形貌和晶体结构进行测试,并对多孔陶瓷的线收缩率、体积密度、显气孔率和抗弯强度进行表征,系统地研究硅溶胶添加对氧化铝多孔陶瓷高温烧结特性的影响.结果表明:低温下硅溶胶的热解产物石英型SiO2将氧化铝颗粒黏结起来,形成物理黏结,能提高多孔陶瓷的力学性能;烧结温度达1500℃时,SiO2开始与氧化铝反应形成莫来石,莫来石结合相的生成使得氧化铝多孔陶瓷趋于致密,力学性能优异,抗弯强度可达(105.5±8.0)MPa;随烧结温度的升高莫来石生成量增多,导致氧化铝多孔陶瓷的体积膨胀,进而使得孔隙率增大,力学性能降低.烧结温度介于1400～1500℃之间时,可以得到微观结构合理、力学性能优异、孔隙率适中的氧化铝多孔陶瓷.     

Static and dynamic flexural strength of 99.5% alumina: Relation to surface roughness

The dynamic flexural strength of ceramics is an important property for all applications involving impact loading conditions. Therefore, this work reports a systematic comparison of static and dynamic flexural strength results for 99.5% commercial alumina, obtained using a recently reported adaptation of the 1-point impact experimental technique. Specimens of the same size and systematically varying surface roughness conditions were used in this study to assess the influence of the latter on the static and dynamic strength of this material. The investigated roughness levels ranged from 0.8 μm (coarse) to 0.05 μm (fine, polished). Under static loading, reducing the surface roughness causes a 10% increase in flexural strength for polished specimens. By contrast, the dynamic flexural strength is apparently not influenced by the surface roughness. A thorough microstructural and fractographic examination reveals the presence of bulk (surface) pore-like flaws that are not obliterated by the polishing process and therefore govern the failure process. It is suggested that strength improvements can be reached by suitable surface preparation, provided no bulk pores are native in the material, some of which are present on its surface. The identification of the role of surface flaws is expected to clarify the discrepancy found in the literature as to the influence of surface roughness on the mechanical properties of brittle materials.     

纸张表面粗糙度和照射条件对纸张白度的影响

用基于测量仪器和样品散射特性的数学模型研究了纸张白度和纸张表面粗糙度的关系,比较了有和没有散射光中的镜面成分条件下的纸张白度和纸张表面粗糙度的关系,纸张白度是根据ISO/DIS 11475计算获得的.结果发现,所研究的纸张白度取决于纸张的表面粗糙度和所选择的照射条件,而纸张的色度坐标与两者无关.     

Pore size distribution,grain growth,and the sintering stress

The effects of a pore size distribution and of the pore separation on the sintering stress is examined using a simple model. The sintering stress is found to be proportional to the mean of the pore sizes weighted according to the Voronoi cell pertaining to each pore, rather than to the simple pore size average. Large heteropores are shown to have little effect on the mean effective sintering stress. Decreases in pore coordination number of such pores, resulting from grain growth can significantly increase the stress intensification factor. The near-constancy of the sintering stress, observed experimentally for many powders over a wide range of sintered densities, does not directly follow from the simple model. It is argued that this constancy results from pore shrinkage, due to densification, which is compensated by pore growth due to coarsening.